The electron microscope was developed to enable imaging of objects and substrates which are too small to be imaged by light microscopes, due to the length of the light wave. The first electron microscopes were developed in the early 1930's and were limited in their resolution by problems such as specimen heating and specimen damage due to high electron energy exposure in general. Considerable work was needed to produce a proper condenser, polepieces for objective and projective, as well as airlocks for specimens and photoplates.
Today, the scanning electron microscope is widely used, mainly for the study of surfaces as well as transparent specimens. Two major applications for the scanning electron microscope are analytical inspection and lithography.
Krans et al., in U.S. Pat. No. 6,218,664 B1, issued Apr. 17, 2001, describe a scanning electron microscope (SEM) provided with an electrostatic objective and an electrical scanning device. The design for the particle-optical apparatus disclosed includes a particle source for producing a primary beam of electrically charged particles which travel along an optical axis of the apparatus towards a substrate/specimen to be irradiated. The primary beam is focused using electrostatic electrodes, to provide a focus point which is in the vicinity of the specimen to be exposed to radiation. A beam deflection system located between the source of the primary beam and the electrostatic electrodes is used to deflect the primary beam so that the beam can be rapidly scanned over the surface which is to be analyzed. The detection means has a longitudinal axis which is essentially perpendicular to the longitudinal axis of the source of the primary beam, which travels through a bore present in the detection means. The design is said to provide advantages in terms of reducing imaging error regardless of the magnitude of the scanning motion of the primary beam.
U.S. Pat. No. 6,320,194 B1 to Khursheed et al. describes a “portable” high resolution scanning electron microscope column using permanent magnet electron lenses. FIG. 3 in the '194 patent illustrates the SEM column, which includes a condenser lense which provides demagnification of the electron beam using two cylindrical shaped permanent magnets. The condenser lense has a center bore for passage of an electron beam from an electron beam gun. Two cylindrical coils are positioned around a pole piece cylinder, through which the primary electron beam passes. Electric current is passed through the coils for the purpose of adjusting the level of demagnification/condensing of the primary electron beam's spot size. The SEM column also includes an objective lens. The objective lens includes a tapered objective lens pole piece structure including a cylinder having a bore through which the electron beam passes. The objective lens is also designed to generate the magnetic field primarily through use of a permanent magnet or magnets which are positioned as far away from the pole piece as is practical. The permanent magnet is used in combination with a tuning coil which is used to adjust the focus of the beam on the specimen. The description in the Khursheed et al. patent illustrates a compact electron optics, but does not address the mobility of the electron microscope system which is significantly affected by elements other than the electron optics. The mobility of the system as a whole is particularly important for use at locations which are difficult to access.
In an article entitled “Electrostatic einzel lenses with reduced spherical aberration for use in field-emission guns”, J. Vac. Sci. Technol. 15 (3), May/June 1978, G. H. N. Riddle describes focusing properties and aberration coefficients for electrostatic einzel lenses suitable for use as preaccelerator lenses in field-emission electron guns. Various lens shapes are analyzed, and asymmetric designs with conical central electrodes are found to have reduced spherical aberration. A lens shape with optimized geometry is found to have a spherical aberration coefficient of less than six times the working distance from the lens to the focal point. The article describes trade-offs between spherical and chromatic aberration, depending on factors such a beam current required, the current/solid angle which is drawn from the emitter, and the voltage spread in the beam. The latter factor is said to depend primarily on emitter temperature, which to a large extent, is determined by vacuum characteristics at the tip of the emitter. Again, this reference focuses on miniaturized electron optics but does not describe an entire system which is sufficiently mobile to be independently used at locations which are difficult to access.
In a presentation at the 1999 NASA/JPL Workshop on Miniature Vacuum Pump Technology, John L. Callas of the Jet Propulsion Laboratory described capabilities for a scanning electron microscope which might be used to sample and analyze specimens from a surface on the planet Mars. A comparison was made of a system which included emerging microcolumn technology and more standard SEM technology. However, the overall system requirements, such as power supply and pump down requirements for this system were considerably different from the requirements for most applications useful on earth. In addition the sample stage described was inadequate for use in a portable scanning electron microscope.